The present invention relates to a thermal processing apparatus.
In a conventional type of thermal processing apparatus, it is common to use a method of maintaining a gas-tight seal that involves placing an O-ring as a sealing means thereof between a flange portion at an opening end of a processing vessel (which is evacuated by means of a vacuum pump) and a lid that closes the opening end.
Other methods have been proposed, such as that described in Japanese Utility Model Laid-Open Publication Number 123336/1989 wherein a double O-ring is used and the space therein is evacuated.
However, with the former method in which an O-ring is used as a sealing member, the O-ring is made of a flexible material such as rubber but it must also withstand temperatures that are generally in the region of 200.degree. C. If the seal portion of the thermal processing apparatus is subjected to temperatures higher than this, the O-ring will melt and deform and thus the prescribed sealing effect will not be obtained. In this case, the vicinity of the O-ring is cooled in order to protect the O-ring. Since it is necessary to secure sufficient flat heating zone for the thermal processing region, this method causes problems in that the processing vessel becomes longer and thus the thermal processing apparatus is bigger overall.
In addition, when the thermal processing apparatus is halted to change the O-ring, the O-ring could stick to the sealing portion when it has cooled down to room temperature, making it difficult to remove. This problem also affects components such as the quartz tube that forms the processing vessel which could become broken while they are being removed.
There is also a danger that the high-temperature heating produced during the thermal processing could cause any gases and moisture included within the O-ring to be emitted. The quantities of such emissions vary with pressure, timing, and temperature. Therefore, if thermal processing is performed on an object to be processed in a processing vessel in which an O-ring acts as sealing portion, a further long period of time is required after a prescribed pressure is achieved to ensure that gases and moisture are fully emitted from the O-ring that provides the vacuum seal. This raises a problem in that this can cause large variations between different processing lots. This need to provide extended gas emission times also degrades the throughput.
With reduction thermal processing that uses a processing gas such as monosilane (SiH.sub.4) to remove natural oxide films from objects to be processed, gases and moisture that cannot be ignored from the processing point of view are emitted from the O-ring, even if a vacuum pump is used to ensure sufficient removal of gases. This causes a problem in that the prescribed reduction thermal processing temperature cannot be obtained.
In another method providing a seal that enables a reduced-pressure environment by using an exhaust means to suck out the atmosphere, a thin annular plate is welded over an opening portion of an annular groove portion when a flange portion is to be sealed (as described in, for example, U.S. Pat. No. 5,133,561), but this method has a problem in that this welding takes time.